The present invention relates to a valve driving apparatus for engines. More particularly, the present invention pertains to a valve driving apparatus that varies performance of a set of intake valves and a set of exhaust valves in an engine according to the operating conditions of the engine by changing the positions of valve actuating cams.
Existing engines have valve driving apparatuses with low speed cams and high speed cams, which have different profiles, provided on an intake camshaft or an exhaust camshaft. The apparatus switches between the low speed cams and the high speed cams in accordance with the operating conditions of the engine thereby changing the valve timing or the valve lift of the intake valves or the exhaust valves. Using two sets of cams having different profiles, the apparatus makes the maximum lift amount of the valves relatively small when the engine speed is low and makes the maximum valve lift amount of the valves relatively large when the engine speed is high. In this manner, the apparatus guarantees optimum engine characteristics such as torque and stability both in the low speed range and in the high speed range of the engine.
FIG. 12 shows a valve driving apparatus of another type used in an engine having four valves per cylinder. This apparatus is provided on a camshaft 42 (either the intake or exhaust camshaft of the engine), which is supported by a bearing 44. Cams 40 are fixed on the camshaft 42. A pair of the cams 40 corresponds to a pair of valves 43 (either intake or exhaust valves) located in an engine cylinder. Each cam 40 is a solid cam having a surface 40a. The cam nose radius of each cam 40 continuously varies in the axial direction of the camshaft 42. The cams 40 are integrally moved with the camshaft 42 in the axial direction (to the left or the right in the drawing) by a shaft moving mechanism 41. This changes the effective cam nose radius of the cams 40.
The range of change of the maximum lift amount (hereinafter, referred to as the lift control amount) is determined according to the difference between the maximum value and the minimum value of the radius of the cam nose. The axial position of the cam shaft 42 is controlled such that the maximum lift of the valves 43 is small in the low engine speed range and is large in the high engine speed range. Therefore, the apparatus of FIG. 12 optimizes engine characteristics such as the torque and stability both in the low speed range and in the high speed range of the engine.
A valve lifter 49 is located between each valve 43 and the corresponding cam 40. A cam follower 45 is pivotally located on top of each valve lifter 49. The surface 45a of the cam follower 45 slidably contacts the cam surface 40a. The cam follower 45 pivots as it slides on the cam surface 40a. That is, the surface 45a of the cam follower 45 functions as a sliding surface that slides on the cam surface 40a.
In such an engine having four valves per cylinder, the bearing 44 must be located between a pair of cams 40 that correspond to a single combustion chamber for ensuring sufficient rigidity of the camshaft 42. Also, the distance between the valves 43 is determined in accordance with the size of each combustion chamber and cannot be widened. The axial moving amount D of the cams 40 is therefore limited to avoid interference between the cams 40 and the bearing 44. Further, the size of the combustion chamber, that is, the distance between the adjacent valves 43 limits the axial moving amount D of the cams 40. The limited axial moving amount D of the cams 40 corresponds to an insufficient range of valve performance variation, or an insufficient lift control amount of the valves 43.
For increasing the lift control amount in an engine having four valves per cylinder, Japanese Unexamined Patent Publication 3-179116 discloses another type of valve driving apparatus. This apparatus includes a single valve lifter for actuating a pair of valves. FIG. 13 shows a partial cross-sectional view of the apparatus.
The apparatus includes a single cam 51 and a single valve lifter 59 that correspond to two valves 58. The two valves 58 are actuated by the single cam 51 through the single valve lifter 59. This construction increases the width W the cam 51 and the axial moving amount D of the cam 51 compared to the apparatus of FIG. 12 without changing the inclination angle .theta. of the cam nose. Accordingly, the lift control amount is increased.
As shown in FIG. 14, the valve lifter 59 is shaped like a rectangle with rounded ends when viewed from above. In other words, its side surface has an oblong shape. Accordingly, the bore formed in the cylinder head for accommodating the lifter must also be shaped like a rectangle with rounded ends. Therefore, compared to circular valve lifter, it is difficult to obtain the required dimensional accuracy of the valve lifter 59. Further, the valve lifter 59 supports two valves 58 at predetermined positions. This complicates the construction of the valve lifter 59. Further, the valve lifter 59 and the corresponding oblong lifter opening are larger than a valve lifter that actuates a single valve and its corresponding lifter opening. Therefore, it is difficult to achieve the required assembly tolerances for the valve lifter 59 and the corresponding lifter opening. Hence, the manufacture of the valve lifter 59 and the engine is significantly complicated.
Methods to increase the lift control amount without changing the width W of cams and the moving amount D of the cams include increasing the inclination angle .theta. of the cam surface 40a for increasing the difference between the maximum value and the minimum values of the radius of the cam nose. However, increasing the inclination angle .theta. of the cam nose increases force required for moving the cam shaft 42 to the right in FIG. 12. In order to gain the sufficient force to move the camshaft 42, the valve moving apparatus 41 needs to be enlarged.
Another method is to decrease the width S of the sliding surface 45a of each cam follower 45. This increases the effective length of the cam surface 40a on which the cam follower 45 moves. However, decreasing the width S of the sliding surface 45a increases the pressure acting on the sliding surface 45a. The increased pressure accelerates the wear of the cam follower 45 thereby drastically reducing the durability of the cam follower 45.